Pollution control devices are used on motor vehicles to reduce atmospheric pollution. Such pollution control devices include one or more pollution control elements mounted within a housing. Two types of devices are currently in widespread use: catalytic converters and diesel particulate filters or traps. Catalytic converters contain one or more catalysts, which are coated onto a substrate typically in the form of a monolithic structure. The monolithic structures are typically ceramic, although metal monoliths have been used. The catalyst(s) oxidize carbon monoxide and hydrocarbons, reduce the oxides of nitrogen in exhaust gases, or a combination thereof. Diesel particulate filters or traps typically include a wall flow filter element in the form of a honeycombed monolithic structure made from porous crystalline ceramic materials. In the current state-of-the-art construction of these pollution control devices, the monolithic structure of each type is enclosed within a housing.
A typical monolithic pollution control element generally has relatively thin walls to provide a large amount of surface area. Consequently, the structure is relatively fragile and susceptible to breakage. The typical monolithic pollution control element formed from ceramic material tends to have a coefficient of thermal expansion that is an order of magnitude less than the metal (usually stainless steel) housing or can in which it is contained. Protective packing or mounting materials such as intumescent or non-intumescent mats are typically packed between the ceramic monolith and the metal housing to avoid damage to the monolith from, for example, road shock and vibration, to compensate for the thermal expansion difference, and to prevent exhaust gases from passing between the monolith and the metal housing. The process of placing or inserting the mounting material is referred to as “canning” and includes such processes as injecting a paste into a gap between the monolith and the metal housing, or wrapping a sheet material (i.e., mounting mat) around the monolith and inserting the wrapped monolith into the housing.
Compositions used to form conventional mounting materials have included refractory ceramic fibers that can provide properties such as high temperature durability, good handling, resiliency, flexibility, and strength. Intumescent materials have also been used that volumetrically expand at elevated temperatures. Such expansion can help to hold the monolith in place during use at high temperatures. Materials that have been used for mounting pollution control elements in the housing of a pollution control device are described in, for example, Published German Patent Application No. DE 19858025 (Asglawo); and U.S. Pat. Nos. 3,916,057 (Hatch et al.); 4,305,992 (Langer et al.); 4,385,135 (Langer et al.); 5,254,410 (Langer et al.); and 5,242,871 (Hashimoto et al.).
Relatively small refractory ceramic fibers, i.e., those having a diameter of less than about 5 to 6 micrometers, have been an important component of mat compositions. However, small fibers may be troublesome in some instances. Small fibers are deemed to be respirable and are usually durable in physiological fluids, in particular, lung fluids. Thus, there is a desire for mounting mat compositions in which the use of durable, respirable refractory fibers could be avoided. It has been a significant engineering challenge, however, to make an acceptable mounting mat without this component.
Fibers that are non-durable in physiological fluids have been described in various references including U.S. Pat. No. 5,874,375 and described references therein. When such a fiber is inhaled, it decomposes in body fluids. Unfortunately, soluble inorganic fibers by themselves are not a practical substitute for refractory fibers. This lack of interchangeability can be attributed to faults that burden soluble inorganic fibers. Such fibers tend to exhibit excessive shrinkage when subjected to the temperature extremes encountered in a typical catalytic converter. Mat shrinkage can allow a fragile catalytic converter element to be loosely held inside its housing resulting in potential damage to the monolith and thus the pollution control device.
There is a continuing need for improvements to such mounting mat compositions. The present invention provides such improved compositions.